This invention relates to a method and apparatus for metering a foamed material in a uniform, controlled manner; most preferably for the purpose of applying the material uniformly across the width of a fibrous assemblage.
The dispensing of stable and non-stable foamed liquids onto a moving surface from an applicator unit is known in the art; as is exemplified by U.S. Pat. Nos. 3,181,199, issued to Voelker, and 3,395,201, issued Kalwaites. Although the properties of these two types of foamed liquids are very different from each other, they are both difficult to disperse as thin, uniform layers across the width of a moving surface. This is particularly troublesome with non-stable foams that tend to loose their cellular structure upon engaging the moving surface, since there is very little opportunity to doctor these foams into a uniform layer. The problem is most acute when the moving surface receiving the non-stable foam is a fibrous web, which is the case in the bonding operation described in the above-mentioned Kalwaites patent. In this latter situation the capillary properties of the fibrous web tend to aid in very rapidly breaking down the foam structure by drawing liquid from the foam into the web surface.
In accordance with the foam bonding technique disclosed in the above-mentioned Kalwaites patent, nozzles are employed to direct a foamed binder into nips provided between compression rolls and the fibrous web to be bonded. The nozzles will initially direct the foamed binder into engagement with the web in an uncontrolled manner, and thereafter, the binder will be forced into the web by the compression rolls. Even though the compression rolls will provide some metering of the binder, the Kalwaites technique is not believed to be well suited for use in obtaining a uniform cross-machine-direction distribution of binder on the web; especially if the web is wide. This lack of uniformity can create undesirable variations in product properties.
U.S. Pat. No. 3,181,199, issued to Voelker, is directed to an applicator for both generating a stable froth, such as of polyurethane, and thereafter directing the froth onto a moving surface in the form of a thick, stable foamed sheet. Voelker does not in any way relate to a method of metering a thin, uniform layer of a foamed material onto his moving surface. Clearly, Voelker does not even touch upon the special problems associated with achieving a uniform lateral distribution of non-stabilized foamed materials onto moving surfaces; especially onto fibrous webs where cell break down and liquid absorption tend to occur very rapidly.
In view of the state of the art, a need is believed to exist for an improved foamed liquid application method and apparatus for achieving a uniform cross-machine-direction application of a foamed material onto a moving surface without the need for doctoring the material, and, when the moving surface is a fibrous web, without the need for significantly compressing the web. It is to such a foamed liquid application method and apparatus that the present invention is directed.